/Engine/Plugins/Animation/RigLogic/Source/RigLogicLib/Public/dna/layers/GeometryWriter.h
Both the mesh storage itself and it's blend shape target storage will be implicitly resized (if needed) to provide storage for the number of meshes and/or blend shape targets that are inferred from the specified indexes.
void setBlendShapeTargetDeltas ( std::uint16_t meshIndex, std::uint16_t blendShapeTargetIndex, const [Delta](API\Plugins\RigLogicLib\dna\layers\Delta) * deltas, std::uint32_t count )